Allergies can develop over time - the thing people don't realise is that it's usually something they've been exposed to many times before, not something new. Sensitivities can build from *too much* or just regular exposure. A lot of people develop sensitivities to things like latex or acrylic from their jobs and such.
I got full-on body-covering hives in spring for a few years. Instead of trying to pin down exactly what Spring Thing was causing them, they gave me a round of steroid pills and I was okay.
It is getting to be that time of year, so it could be that.